Are the Always ultra thin overnight pads good for heavy flows? And how many hours can it take for heavy flows?

by definitions, you should change your pad every 8 hours. So, go change into a new one when you go to bed, and by the time you wake up, replace with a new pad.

Sometime, you would be awaken during the night if it is too heavy. Just change it when you know about. I always lay an old shirt or old towel on the sheet in case these bloody fluid overflows.